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Long-term debt - Narrative (Details)
 2 Months Ended 12 Months Ended
Mar. 06, 2018
USD ($)
vessel
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Disclosure of detailed information about borrowings [line items]        
Net proceeds on issue of long-term debt   $ 0 $ 65,700,000  
Accretion of deferred financing costs   3,100,000 3,000,000  
Cash balance   375,479,000 $ 223,890,000 $ 254,934,000
Egypt entity        
Disclosure of detailed information about borrowings [line items]        
Cash balance   $ 131,000,000    
Egypt entity        
Disclosure of detailed information about borrowings [line items]        
Ownership basis, percent   100.00%    
Egypt limited recourse debt facilities        
Disclosure of detailed information about borrowings [line items]        
Facilities term   12 years    
Egypt limited recourse debt facilities | Egypt entity        
Disclosure of detailed information about borrowings [line items]        
Percentage of gas requirements (greater than)   70.00%    
Shareholder distributions subject to restriction   $ 100,000,000    
Principal repayments to be matched with shareholder distributions   $ 100,000,000    
Egypt limited recourse debt facilities | Minimum | LIBOR        
Disclosure of detailed information about borrowings [line items]        
Basis spread, percent   0.90%    
Egypt limited recourse debt facilities | Maximum | LIBOR        
Disclosure of detailed information about borrowings [line items]        
Basis spread, percent   1.60%    
Other limited recourse debt facilities | Issuance of debt        
Disclosure of detailed information about borrowings [line items]        
Notional amount $ 43,000,000      
Number of vessels to be acquired | vessel 2      
Other limited recourse debt facilities | Fixed interest rate | Issuance of debt        
Disclosure of detailed information about borrowings [line items]        
Borrowings, interest rate 5.35%      
Other limited recourse debt facilities | 50% owned subsidiary | Issuance of debt        
Disclosure of detailed information about borrowings [line items]        
Interest % 50.00%      
Notional amount $ 86,000,000      
Other limited recourse debt facilities | Minimum        
Disclosure of detailed information about borrowings [line items]        
Basis spread, percent   0.75%    
Facilities term   2 years    
Other limited recourse debt facilities | Maximum        
Disclosure of detailed information about borrowings [line items]        
Basis spread, percent   2.50%    
Facilities term   4 years    
Revolving credit facility        
Disclosure of detailed information about borrowings [line items]        
Facilities term   5 years    
Notional amount   $ 300,000,000    
Debt covenant, EBITDA to interest coverage ratio   2    
Debt covenant, debt to capitalization ratio (less than or equal)   55.00%    
Default payment accelerated by a creditor, amount of indebtedness (or more)   $ 50,000,000    
Default that permits a creditor to demand repayment, amount of indebtedness (or more)   $ 50,000,000